1. Field of the Invention
The present invention relates to an electrode arrangement of a vacuum circuit breaker having improved breaking characteristics, and in particular to an electrode arrangement of a vacuum circuit breaker having a magnetic member for generating a longitudinal magnetic field between a pair of contact members for making electric connection and break.
2. Description of the Prior Art
A vacuum circuit breaker normally comprises, as shown in FIG. 1, a vacuum container 1 having an insulating container 2 with both end opening portions thereof being closed by covers 3a and 3b, and a pair of electrodes. The paired electrodes comprise contacts 4 and 5 which are arranged to face each other in the vacuum container 1 and conductive bars 6 and 7 which pass through the covers 3a, 3b and inserted into the vacuum container 1, respectively. The contacts 4 and 5 are provided on the end portions of the conductive bars 6 and 7, respectively. One conductive bar 7 is movable in the axial direction by an operation mechanism (not shown) such that one contact (to be referred to as "fixed contact" hereinafter) 4 can contact with and release from the other contact (to be referred to as "movable contact" hereinafter) 5.
A bellows 8 is provided between the cover 3a and the conductive bar 7 to tightly hold vacuum the inside of the vacuum container 1 and to allow the conductive bar 7 to move in the axial direction. Reference numeral 9 denotes a shield provided so as to surround the contacts 4 and 5 as well as the conductive bars 6 and 7.
The vacuum circuit breaker is normally energized when both of the contacts contact with each other. In this state, when the conductive bar 7 moves in the direction indicated by an arrow M, the movable contact 5 separates from the fixed contact 4 and an arc is generated between the contacts 4 and 5. The arc is maintained by generating a metallic vapor from a cathode such as a movable contact 5. As the contacts are distant from each other, the arc cannot be maintained, no current flows, and the generation of the metallic vapor stops to thereby complete breaking.
The arc generated between the contacts 4 and 5 turns into an extremely unstable condition by the interaction between a magnetic field generated by the arc itself and a magnetic field generated by an external circuit if the current to be broken is high. As a result, the arc moves on surfaces of the contacts and is biased to end portions or peripheral portions of the contacts. These arced portions are locally heated and a large quantity of metallic vapors are discharged, so that the degree of vacuum in the vacuum container 1 is thereby lowered. The breaking characteristics of the vacuum circuit breaker thus deteriorates. If the contacts are integrally formed on the electrodes, the arc may move on surfaces of the electrodes.
To avoid the deterioration of the breaking characteristics, there have been proposed, for example, (a) an electrode structure in which the contact surfaces have larger areas; (b) that in which a spiral slit is provided on the surfaces of the contacts or on the surfaces of the electrodes to rotate the arc; and (c), as shown in FIG. 2, a longitudinal magnetic field parallel to the arc is applied to the gap between the contacts by means of circumferential components of self-current which flow coil electrodes 10 and 10' being provided on the back of the contacts 4 and 5, respectively.
In a case of the electrode structure of (a) above, a biased arc may still be generated as described above. As a result, the contacts (electrodes) are locally molten and a vapor is generated more, whereby it may make circuit breaking impossible.
In a case of the electrode structure of (b) above, it is also impossible to uniformly flow current across the entire areas of the contacts, with the result that the phenomenon as same as the case of (a) occurs.
In a case of the electrode structure of (c) above, if current flows across the coil electrodes on the back of the contacts, a magnetic field is generated between the contacts in a direction perpendicular to the contact surface. During breaking operation, the arc generated between the both contacts is restricted by the longitudinal magnetic field. The arc distribution becomes the same as that of the line of magnetic force between the contacts. However, the distribution is not necessarily uniform and parallel. In addition, there occurs a phenomenon that the arc does not strike perpendicular to the contact surface and even shifts from the space between the contacts to the outside in the vicinity of the end portions of the respective contacts, with the result that expected breaking characteristics may not be exhibited.
As stated above, various improvements have been tried so far to contacts as well as electrode structures having the contacts provided thereon. Some of them, however, provide insufficient breaking characteristics and others push up cost.